1. Field of the Invention
The invention relates to a method of determining a characteristic of an optical fiber by reflectometry.
2. Description of the Prior Art
The bit rate at which information is transmitted by telecommunication networks is continually increasing. Such networks are therefore using increasing quantities of optical fibers able to satisfy this demand for high bit rates.
The characteristics of each optical fiber fabricated must be verified to assure that they conform to the specifications. This entails various types of measurement effected using various systems and necessitating many manipulations of the fiber. Clearly it would be preferable to have measuring systems simultaneously supplying reliable results for the greatest possible number of characteristics.
The systems most widely used consist of reflectometers or backscattering measuring systems. They are used to determine the attenuation of monomode fibers and their mode diameter. The mode diameter represents the diameter of a cylinder within which most of the energy propagates.
Reflectometry is based on the fact that an optical fiber diffuses light in all directions, including backwards. Thus a light pulse of power P0 emitted at time t=0 at one end of the fiber travels a distance L in a time t1, and at the end of this time t1 its power is P0exe2x88x92xcex1L, and at the end of a time 2t1 a pulse of power xcex7P0exe2x88x922xcex1L is collected at the same end of the fiber, where xcex1 is the attenuation of the fiber and xcex7 is its backscattering factor.
The backscattering factor is (among other things) inversely proportional to the square w2 of the mode diameter w.
To perform the measurement, the first end of the fiber to be characterized is connected to a first reference fiber (or standard fiber) and the second end of the fiber to be characterized is then likewise connected to a second reference fiber.
The power is measured at the free ends of the reference fibers. Each measurement entails determining the value of the backscattered power at various points on the fiber to be characterized and the reference fibers. One measurement is performed from the free end of the first standard fiber and another measurement is performed from the free end of the other standard fiber. For example, the mode diameter is determined by representing the two measurements so that the abscissae L have the same origin in both cases and by considering the sum of the logarithms of the backscattered powers at each point. At a given point with abscissa x, one measurement supplies xcex7P0exe2x88x922xcex1x and the other measurement supplies xcex7P0exe2x88x922xcex1(L1xe2x88x92x), where L1 is the sum of the lengths of the reference fibers and the fiber to be characterized. It is found that the sum of the logarithms is independent of the abscissa x.
The above measurement process is particularly efficient. However, it is highly sensitive to misalignments between the reference fibers and the fiber to be characterized. This is because the small diameters of monomode fibers makes them very difficult to manipulate and align. Also, vibrations at the splices cause measurement errors. To solve these problems a plurality of measurements is taken. However, this solution is clearly not satisfactory because it is time-consuming and is therefore costly in terms of labor.
The invention solves these problems.
The invention provides a method of determining at least one characteristic of an optical fiber using a reflectometer, wherein the fiber to be characterized is connected at both ends to reference fibers, a first reflectometry measurement is performed using the first reference fiber and a second reflectometry measurement is performed using the second reference fiber, to determine if the reflectometer data relating to the fiber to be characterized obtained after the above two measurements is correct or not a reflectometer data value at one point at least of the first reference fiber is compared to a reflectometer data value at one equivalent point at least of the second reference fiber and the reflectometer data for the fiber to be characterized is retained only if the difference between the compared data values is the same for both measurements.
In the preferred embodiment of the invention, the compared reflectometric data values of the two reference fibers are mean values for the same lengths of the reference fibers.
More generally, the compared reflectometric data values of the two reference fibers are mean values for equivalent segments of the reference fibers and two segments are equivalent if they consist of a set of equivalent points.
This method is particularly simple, quick and reliable, in particular when the reflectometric data concerned is obtained by adding the power detected during the first measurement and the power detected during the second measurement logarithmically for each abscissa, the length origins being the same for both measurements.
In one embodiment of the invention the reference fibers have practically identical characteristics.
The reflectometric data is used to determine the mode diameter of the fiber to be characterized, for example.
The reflectometric data can also be used to determine the chromatic dispersion of the optical fiber from reflectometric data obtained at two wavelengths at least.
In one embodiment of the invention the chromatic dispersion is obtained from the following equation:             D      ⁢              (        λ        )              =                            C          0                ⁢                  (          λ          )                    +                        ∑          i                ⁢                                            C              i                        ⁢                          (              λ              )                                ·                      x            ⁢                          (                              λ                ⁢                                  xe2x80x83                                ⁢                i                            )                                            ,
in which D(xcex) is the chromatic dispersion at the wavelength xcex, x(xcexi) is a reflectometric data value corresponding to a measurement at a wavelength xcexi and C0(xcex) and Ci(xcex) are coefficients dependent on the wavelength xcex.
The coefficients C0 and Ci are obtained empirically, for example.
In one variant of this latter embodiment, the coefficients C0 and Ci are obtained by means of a plurality of preliminary measurements performed on a series of fibers using chromatic dispersion measuring systems and a reflectometer, the coefficients C0 and Ci being thereafter determined from those measurements by linear regression.
In one embodiment of the invention x(xcexi) has the following value:
x(xcexi)=(ymxe2x88x92yA)xcexi,
in which equation ym is a mean reflectometric data value at the wavelength xcexi of the fiber to be characterized and yA is the same mean reflectometric data value at the same wavelength xcexi for a reference fiber.
In another embodiment of the invention x(xcexi) has the value:       x    ⁢          (              λ        ⁢                  xe2x80x83                ⁢        i            )        =                    L                  fopt          ⁢                      (                          λ              ⁢                              xe2x80x83                            ⁢              i                        )                              -              L                  fopt          ⁢                      (                          λ              ⁢                              xe2x80x83                            ⁢              0                        )                                      L              fopt        ⁢                  (          λ0          )                    
where xcexi is the measurement wavelength and Lopt(xcexi) is an optical length such that:             L      opt        ⁢          (              λ        i            )        =            c              2        ⁢                  n          ref                      ⁢          T      ⁢              (                  λ          i                )            xe2x80x83Lfopt(xcexi)xe2x88x92=Lopt(xcexi)xe2x88x92L0opt(xcexi),
where c is the speed of light, nref is the reference refractive index of the fiber to be characterized, T(xcexi) is the transit time, measured by reflectometry, corresponding to twice the transit time between one end of a reference fiber and the end of the other reference fiber, and L0opt(xcexi) is the optical length of the reference fibers at the wavelength xcexi.
The transit time T(xcexi) can be determined by seeking the maximum correlation between the reflectometric signature observed for the end of the other fiber and the expected signature for that end.
For example, the maximum correlation is sought in incremental reflectometric data between consecutive data points, preferably expressed on a linear scale.
Other features and advantages of the invention will become apparent from the description of embodiments of the invention given with reference to the accompanying drawings.